Generally, capacitors are employed in integrated circuits to perform a variety of functions. Capacitors can be used to construct band pass filters, phase locked loops (PLLs), dynamic random access memory (DRAM) components, and a host of other useful devices. In some instances, some common elements of an integrated circuit exhibit inherent capacitance.
For example, certain active integrated circuit elements, such as, for example, bipolar and metal-oxide-semiconductor (MOS) transistors, contain electrical junctions that exhibit capacitance. In fact, depending on the particular type of transistor, a depletion region of an electrical junction can be described as functionally equivalent to a small parallel-plate capacitor. Such a capacitor can be modeled as a fixed-value capacitor, or as a variable capacitor, with a capacitance that changes as a function of the voltage applied to the junction. Furthermore, certain passive integrated circuit elements, such as, for example, polycrystalline silicon (polysilicon) and metal lines, also have inherent capacitance with respect to each other and to any other conductors.
One disadvantage, however, in employing such inherent capacitance to achieve certain functionality, is that the inherent capacitance is often insufficient and difficult to engineer. For example, because this inherent capacitance is typically a byproduct of some other functionality for which the particular element is designed, the inherent capacitance cannot be a higher design priority than the primary functionality. Moreover, because inherent capacitance is tied to a particular element, the capacitive effect is tied to that element's location in a circuit and cannot be relocated. Thus, integrated circuits often employ dedicated capacitors as circuit elements in their own right.
Traditional capacitors are two conductive materials separated by a dielectric. In integrated circuits, the two conductive materials are often flat plates with an intervening layer of dielectric material. One significant disadvantage of this approach, however, is that a relatively large area of the integrated circuit chip is typically required to achieve the desired capacitance.
One structure employed to increase capacitance is a metal-insulator-metal (MIM) capacitor. In its simplest configuration, a number of horizontal parallel plates of metal are stacked into several layers, separated by dielectrics. The plates are conductive and alternately coupled to form opposite electrodes of a capacitor. The vertical stack of plates is simple to construct, and offers more capacitance per unit area than two conductive surfaces alone. However, while simple to construct, forming a MIM capacitor with many layers often requires additional processing steps, which can add prohibitive cost to the manufacturing process.
Another structure employed to increase capacitance is a metal-oxide-metal (MOM) capacitor. Generally, MOM capacitors consist of strips of conductive material of opposite polarity separated by dielectric material. MOM capacitors can often take advantage of existing process steps. For example, the dual-damascene techniques typically used with copper multilevel interconnection metallization on integrated circuits can be used to construct stacks of copper-filled vias and trenches. Two or more such copper-filled vias or trenches, separated by oxide dielectrics, can form a MOM capacitor. MOM capacitors offer greater capacitance per unit area than traditional capacitors, with an efficient form. However, MOM capacitors also typically require a complex design, which can overcome benefits gained by taking advantage of standard semiconductor device manufacture process steps.
Some modern methods employ both MOM and MIM capacitors. However, typically these capacitors, when combined, are formed on separate layers of an integrated circuit, with a MIM capacitor layer stacked above a MOM capacitor layer. Thus, while the capacitance is increased, the vertical chip area required is also increased, which can also add complexity to the design and manufacturing process.
Furthermore, some MOM capacitors are formed with vertical stacks of MOM layers. While these stacked MOM capacitors can offer increased capacitance, however, mismatches in alignment between layers can cause uncertainty in the manufacturing process and performance degradation. At the very least, where the stacks are not aligned the actual capacitance deviates from the expected capacitance, which can cause other devices that depend on the capacitor to behave unpredictably, cascading through the chip.
Therefore, there is a need for a system and/or method for forming improved integrated circuit capacitors that overcomes at least some of the disadvantages associated with previous systems and methods.